The reforming of petroleum raw materials is an important process for producing useful products. One important process is the separation and upgrading of hydrocarbons for a motor fuel, such as producing a naphtha feedstream and upgrading the octane value of the naphtha in the production of gasoline. However, hydrocarbon feedstreams from a raw petroleum source include the production of useful chemical precursors for use in the production of plastics, detergents and other products.
The upgrading of gasoline is an important process, and improvements for the conversion of naphtha feedstreams to increase the octane number have been presented in U.S. Pat. Nos. 3,729,409, 3,753,891, 3,767,568, 4,839,024, 4,882,040 and 5,242,576. These processes involve a variety of means to enhance octane number, and particularly for enhancing the aromatic content of gasoline.
Processes include splitting feeds and operating several reformers using different catalysts, such as a monometallic catalyst or a non-acidic catalyst for lower boiling point hydrocarbons and bi-metallic catalysts for higher boiling point hydrocarbons. Other improvements include newer catalysts, as presented in U.S. Pat. Nos. 4,677,094, 6,809,061 and 7,799,729. However, there are limits to the methods and catalysts presented in these patents, and which can entail significant increases in costs.
In general, high operating temperatures are preferred for operating a reformer, as the equilibriums at the higher temperatures favors the formation of aromatic compounds. However, the reforming process is operated at a lower temperature due to the thermal cracking and the metal catalyzed coking that occurs as the temperature is increased. Recent advances however have provided advances that allow for the reforming reactions to be done at increased temperatures. For example, as disclosed U.S. Pat. Pub. No. 2012/0277500, the entirety of which is incorporated herein by reference, it has been found that using reactor vessels with non-metallic coatings allow for higher temperature operations, without the accompanying increase in coking or thermal cracking.
Increasing the temperature would normally be a preferred condition, since the higher temperatures shift the equilibriums of the reforming reactions to favor the production of aromatics. However, increasing the temperatures increases the formation of coke on the catalyst, and more rapidly deactivates the catalyst. This in turn requires more energy to regenerate the catalyst on a more frequent basis. Increasing temperatures also increases thermal cracking for the heavier hydrocarbons, and can start or increase metal catalyzed coking on the surfaces of the reactor vessel or piping used to transport the hydrocarbons to the reformer. The thermal cracking of naphtha feed and intra-reactor reformed product streams lead directly to yield loss through the production of light (one to four carbon) hydrocarbons.
Some reforming processes utilize flow schemes in which high-temperature, short residence time reactors favor ring closure over catalytic cracking and dealkylation reactions. While catalytically beneficial this design also requires increased temperatures in heaters and transfer lines to the terminal reactors giving corresponding increases in thermal cracking yield losses. Although thermal cracking exists in all reforming units the potential for yield loss is potentially higher in such designs due to the elevated temperatures intrinsically required by the process.
Therefore, there remains a need for an effective and efficient process for minimizing the amount of thermal cracking of hydrocarbons in a reforming reaction.